Smaller scale, but at the bottom of the report it says the following. 793 of your troops were injured in the battle! 1326 of them have been reassigned as reinforcements, and are awaiting treatment in the infirmary.

In some kinds of battles, your troops will die. The most common of these is attacking cities and strongholds.

The other way your troops can die is if your infirmary is full when your marching parties come back with wounded. Once your infirmary is full, all other wounded troops will die until you clear it out. Troops that're currently healing don't take up infirmary space.

You likely had troops you needed to heal already and let your infirmary get too full.

I understand this. It's the below explanation that explains what is happening. If I send 1 troop into battle, that troop gets injured, I have 5 reinforcements to heal. Not exact numbers, but it gives me a better estimate of the kind of hospital capacity I need to keep in mind.